The present invention relates to a semiconductor light emitting device. More particularly, the present invention is useful for a semiconductor laser device. Further, the semiconductor light emitting device according to the present invention is useful for use with optical information processing and optical communication light source.
A high output semiconductor laser and a process for forming an element structure thereof for the purpose of a system application of a rewritable optical disk can be seen, for example, in IEEE J. Quantum Electron. 1993, vol. 29, No. 6, pp. 1874-1879, and IEEE Photonics Technol. Lett. 1997, vol. 9, No. 4, pp. 413-415.
On the other hand, it is requested that an injection current (or, an injection amount of a carrier of an electron, a positive hole or the like) to an active layer necessary for oscillation of a laser beam having a fixed intensity be reduced to increase an optical output of a semiconductor laser device. The reduction in a threshold current of the semiconductor laser device is one of techniques to fulfill the aforementioned request.
Further, as a technique for solving a problem of a leakage of a carrier injected into an active layer without contributing to laser oscillation, of a so-called carrier overflow, there is disclosed, in Japanese Patent Laid-Open No. Hei 5-7051, a technique in which a barrier region having a strained super lattice is provided between the active layer and a cladding layer. Further, a technique using a multi quantum barrier (MQB) structure similar to a multi-quantum well structure is disclosed in Japanese Patent Laid-Open No. Hei 6-244509. In these techniques, due to the barrier region or the multi-quantum barrier structure, a carrier which is about to leak from the active layer is returned to the active layer by the barrier region or the multi-quantum barrier structure. The operation of an operating mechanism thereof is performed by a potential barrier formed by the barrier region or the multi-quantum barrier structure. Furthermore, there is disclosed, in Japanese Patent Laid-Open No. Hei 6-334265, the constitution in which the multi-quantum barrier structure is employed as an optical guide layer of a multi-quantum well type active layer in order to adapt the aforementioned techniques to an active layer having the multi-quantum well structure.
While in the conventional techniques disclosed in above-described references and the like, the process for forming a window structure necessary for attaining a high output in terms of characteristics of elements is mentioned but the process simultaneously concurrently holding a low threshold operation other than the change of the active layer structure is not referred to.
Further, the above-described three Japanese Patent Laid-Opens disclose the technique in which the carrier overflow from the active layer is suppressed in the barrier region of the multi-quantum structure or the super lattice structure to realize the low threshold operation of the semiconductor laser, but do not teach whether or not the aforementioned techniques can be applied to the preparation of an element structure on the substrate having an angle of inclination from a main surface of a crystal. Such a substrate having on its surface the surface inclined from the main crystalline surface as a reference will be hereinafter referred to as xe2x80x9cmisoriented substratexe2x80x9d. As the main crystalline surface, the miller index surface, for example, (100) surface can be mentioned.
In use of the misoriented substrate, for example, a semiconductor laser of a AlGaInP group has to be introduced in order that the oscillation wavelength is made into 650 nm or less. In the case where a crystal is grown on the misoriented substrate, the grown surface sometimes exhibits the shape which is different from that of the crystal grown on the normal substrate (the surface of the surface orientation (100) in the zinc blend crystalline structure). As an angle of inclination xcex8 is set larger, a trouble of the photoconductive wave caused by the aforesaid shape, a deformation of a near field image and the like also appear. The problem resulting from the misoriented substrate is desired to be solved in terms of the applied field of the zinc blend type crystal.
A first object of the present invention is to secure a low threshold even if a misoriented substrate is used for a crystal growing substrate to constitute a semiconductor light emitting device.
A second object of the present invention is to provide a new construction capable of concurrently holding a low threshold and a high efficiency operation even if a misoriented substrate is used for a crystal growing substrate to constitute a semiconductor light emitting device. The low threshold and the high efficiency operation are useful matters in designing the structure of an active layer for obtaining a high output characteristic in a laser light source for optical information processing and optical communication system.
The present invention is useful particularly for a semiconductor laser device formed on the misoriented substrate, but attains the low threshold high efficiency operation simultaneously with the high output characteristic of the semiconductor laser device, and further realizes a system device using it as a light source. A semiconductor laser element using the misoriented substrate is widely used for a short-wavelength laser device.
The semiconductor light emitting device according to the present invention has the construction in which as a semiconductor substrate, a substrate away from the (100) surface orientation in the range of an inclination angle xcex8=0xc2x0 less than xcex8xe2x89xa654.7xc2x0 is used, and a semiconductor super lattice period thin film layer is inserted between the misoriented substrate and the active layer region. Further, by making the device described in the following, the quality of a semiconductor crystalline layer provided on the misoriented substrate can be further enhanced, and the element characteristic can be further improved. The present invention can hold simultaneously concurrently the low threshold high efficiency operation of the semiconductor laser element and the high output characteristic by the provision of the means described in detail in the following.
In the substrate away from the (100) surface orientation at the inclination angle xcex8 of 54.7xc2x0, the crystalline surface thereof has the maximum inclination angle that can be actually used in the (111) surface. The inclination of the substrate in excess of the above is not practical.
The details of the semiconductor super lattice period thin film layer according to the present invention will be described later.
Prior to entry into the detailed description of the mode for carrying out the present invention, the main operation and effect of the super lattice periodic thin film layer provided in the semiconductor multi-layer particularly according to the present invention will be explained from three aspects.
A first effect is to enable enhancement of flatness of a multi-quantum well construction active layer. By the present invention, it is possible to keep a low threshold of a semiconductor light emitting device having threshold, for example, a semiconductor laser device. A second effect is to enable realization of a low threshold and high efficiency operation of a semiconductor laser device. A third effect is that the super lattice period thin film layer enables performance of a role as a stop layer of diffusion of impurity diffusion in a semiconductor laser device or the like.
A first aspect is a general aspect for a semiconductor device using a misoriented substrate and a semiconductor light emitting device.
A second aspect is useful particularly for a semiconductor laser device.
A third aspect is useful for a semiconductor light emitting device having a window structure in an optical output portion or a semiconductor laser device and is extremely practical.
First, introduction of a super lattice period thin film layer into the semiconductor multi-layer enhances a flatness and an interface acuteness of a multi-quantum well structure active layer.
Generally, when a misoriented substrate of a semiconductor is used, homogeneity of a crystalline layer provided thereon is enhanced. However, there is a problem in that particularly, in a thin film layer of the quantum well structure, the shake of a film thickness occurs in an order of an atomic layer.
This phenomenon is caused by a difference of a crystal growth speed in a periodic step of a terrace of the (100) surface in the crystalline surface and the atomic layer order. This generates a non-periodic difference in level. This difference in level is normally called a step bunching in this field.
FIG. 1 explains this phenomenon. FIG. 1 shows in its lower portion a schematic view of a section of a laminate of crystal, and schematically shows in its upper portion a valence electron band of a band structure corresponding to the above state. A well structure indicated by the arrow above the lower section shows a valence electron band at a fixed portion.
That is, in a surface 112 of a misoriented substrate 111, a reference surface (100) surface (indicated by a dotted line 117 in FIG. 1) is disposed stepwise (or terraced fields) in a direction of 116. A macroscopic inclination angle xcex8 of the substrate is determined on the basis of the width of a terrace 113 of the (100) surface and a difference in level (a head) 114 between the terraces. A bend line 118 (a dotted line) shown in FIG. 1 shows an omitted portion of a multi-layer. A microscopic unevenness of the substrate surface is succeeded by the growth surface of a semiconductor layer grown thereon to form a difference in level 115 called the step bunching. As can be seen from the schematic view of the valence electron band of FIG. 1, an order 121 which is relatively high in quantum level is positioned in a region corresponding to the terrace 113, whereas an order 120 which is relatively low in quantum level is positioned in a region corresponding to the terrace 114. In the case where the laser oscillation is effected in that state, there occurs the situation in which a laser beam is absorbed by the difference in level 114 between the terraces positioned in the order 120 which is relatively low in quantum level. This leads to a rise of threshold or a fall of light emitting intensity in the semiconductor laser device.
The step bunching appearing on the semiconductor surface grown on the misoriented substrate results in the broad of the quantum level formed in the quantum well layer, or the unevenness of the carrier density confined in the quantum well layer. Accordingly, measures for preventing them are requested.
FIG. 2 is a schematic view showing the fundamental conception of the present invention. FIG. 2 merely shows the portions of a semiconductor multi-layer. Needless to say, various members necessary for a semiconductor laser device such as an electrode, a protective film of a light emitting surface and the like are provided on the semiconductor laminate in order to form a specific semiconductor light emitting device. Further, a semiconductor layer as desired, for example, a buffer layer for improving a crystallinity, and the like can be used as necessary. As shown in FIG. 2, a different-kind double junction structure composed of a plurality of optical waveguide layers 101, 104 (normally termed as a cladding layer) and an active layer 103 sandwiched therebetween, which is a fundamental structure of a semiconductor laser device, is provided on the misoriented semiconductor substrate 100. In the present invention, a multi-period super lattice thin film layer 102 is at least formed on the lower side of the light emitting active layer 103. Normally, a SCH layer 105 is inserted between the active layer 103 and the optical waveguide layer 104. It is noted that the aforementioned xe2x80x9con the lower sidexe2x80x9d means the crystal growth substrate side, that is, the misoriented substrate side.
FIG. 3 is a schematic view of a fundamental energy band structure according to the present invention. A multi-layer of a semiconductor shown in FIG. 3 corresponds to that of FIG. 1, and a multi-period super lattice thin film layer 102 is introduced on the crystal growth substrate side on the lower side with respect to the light emitting active layer 103. The layer 105 is a separate confinement heterostructure layer (normally called a SCH layer), which is not always necessary for the structure of a semiconductor laser device but is often used generally.
The main role of the multi-period super lattice thin film layer is to periodically repeat semiconductor thin film layers different in composition to obtain a multi-period super lattice thin film layer having a desired thickness to thereby suppress and reduce the oscillation of a film thickness stepwise caused by the step bunching as compared with the case of simply forming a semiconductor multi-layer. In the semiconductor thin film layers different in composition descrived above, one period comprises a combination of a layer having a large band gap (composition 1) and a layer having a band gap smaller than the former (composition 2). A number of one periods are multi-layerd to obtain the multi-period super lattice thin film layer.
In other words, the multi-period super lattice thin film layer comprises one kind of buffers which improves the mohorogy of an interface between a well layer and a barrier layer of a multi-quantum well type active layer formed thereon. When the multi-period super lattice thin film layer is introduced, the step bunching produced on the surface of the optical waveguide layer or the surface of the thin film guide layer under the active layer following the step on the misoriented substrate surface is not remained on a first well layer of the multi-quantum well structure active layer without modification, but the flatness or the interface acuteness of the semiconductor layer formed is considerably improved. Particularly, in a semiconductor material system in which an Al element which is slow in migration is contained in the optical waveguide, the guide layer or the quantum barrier layer, the introduction effect of the multi-period super lattice thin film layer is high.
For example, in a light emitting device comprising an AlGaInP system semiconductor crystal suitable for oscillation of a red laser beam, when it is provided on the misoriented substrate, an order arranging structure of a III group element is suppressed to materially enhance the homogeneity of crystals. On the other hand, however, in the crystalline system, particularly a cladding layer is composed of a four-dimensional crystal containing the Al element, and the step bunching tends to occur on the misoriented substrate. Then, when a multi-period super lattice thin film layer GaInP/AlGaInP system with a GaInP mixed crystal which a three-dimensional crystal not containing the Al element is formed and provided on the lower side in the vicinity of the light emitting active layer as described above, the step bunching suppression effect is obtained.
FIG. 4 shows the examined results of the light emitting characteristics from the active layer, in an example in which the multi-quantum well structure active layer and the multi-period super lattice thin film layer shown Embodiment 1 are provided, as one example, in FIGS. 2 and 3, in the semiconductor light emitting device comprising the GaInP/AlGaInp system material. The characteristic example of FIG. 4 indicates that the crystallinity of the active layer could be improved. The semiconductor light emitting device comprising the GaInP/AlGaInp system termed herein is a semiconductor light emitting device represented in a simplified form in which the active layer or the quantum well layer is formed of GaInP, and the cladding layer for confining a carrier and a light and the quantum barrier layer are formed of AlGaInP. As a result of measurement of a light emitting spectrum shown in FIG. 4, a spectrum II in FIG. 4 is obtained from a specimen into which is introduced the multi-period super lattice thin film layer as compared with a spectrum I into which is not introduced it. The light emitting intensity of the spectrum II increases by 4 to 5 times as compared with that of the spectrum I, and the half value width is reduced by 20 to 30%. As a result of observation of a transmission electron microscope image from a section of a specimen, the step bunching is suppressed on the multi-period super lattice thin film layer, and no difference in level of an irregular 2-atomic layer order is seen. As described above, the flatness and the interface acuteness of the quantum well layer were materially improved. With respect to the multi-quantum well structure in the case where the multi-period super lattice thin film layer is introduced, the improvement of the quality of crystal and the enhancement of the light emitting characteristics are clarified whereby an element contributed to the low threshold high efficiency operation of a laser element could be constructed.
FIG. 4 shows one example. In the quantum well layer which secures the flatness of the atom layer order, the relative light emitting intensity generally increases. On the other hand, in the case where a substrate having a step bunching is used, a spectrum is relatively broad and a light intensity is weak.
Secondly, the operation of the semiconductor laser element with the low threshold and high efficiency can be attained while making a good use of the multi-period super lattice thin film layer for designing the active layer.
Since the multi-period super lattice thin film layer is constituted by a super lattice well layer which is large in refractive index and small in forbidden band width, an optical confinement coefficient can be made large, and transportation of electron carriers within the active layer can be adjusted.
FIG. 5 shows the result of computation and comparison of the optical confinement coefficients in the GaInP/AlGaInP multi-quantum well structure as one example. In the figure, the axis of abscissae indicates the thickness of quantum well layers, and the axis of ordinates indicates the optical confinement coefficient. Curve 120 shows the change of the optical confinement coefficient in the case where an optical waveguide path is constituted having three quantum well layers. Curve 121 shows the change of the optical confinement coefficient in an example in which there are three quantum well layers, and the thin film super lattice layer according to the present invention for 10 periods (here, a set of a quantum well layer and a barrier layer is called one period) is provided on the crystalline substrate side of the active layer. Further, curve 122 shows the change of the optical confinement coefficient in the case where an optical waveguide path is constituted having three quantum well layers.
The cladding layer used is suffice to be one in a normal semiconductor laser device.
For example, a multi-period super lattice thin film layer provided with 11 AlGaInP super lattice barrier layers of thickness 1 nm and 10 GaInP super lattice well layers of thickness 0.5 nm is added to the structure of 3 quantum well layers of thickness 5 nm, whereby the optical confinement coefficient can be made large by 0.0051. This corresponds, in the case of only 3 quantum well layers, to the case where the thickness of the quantum well layer is increased by about 0.5 nm. Further, in the case where the thickness of the quantum well layer is not more than about 4 nm, substantially the same optical confinement coefficient as that of the structure of 4 quantum well layers is obtained. In FIG. 5, the characteristic of the structure having 3 quantum well layers is shown by curve 120, the characteristic of the structure having a multi-period super lattice thin film layer is shown by curve 121, and the characteristic of the structure having 4 quantum well layers is shown by curve 122. As described above, even if the characteristics 120 and 121 in FIG. 5 seem to be similar at a glance, a great effect is brought on in designing various semiconductor devices handling the quantum effect.
When a multi-period super lattice thin film layer is provided, an active layer structure can be designed in which a quantum well layer is increased falsely matching for one layer of the quantum well layer. In designing an active layer for reducing an optical density of the active layer to enhance the high output characteristic, there is set to a region where the optical confinement efficiency is relatively small. However, it is advantageous for the low threshold high efficiency operation of elements to make the optical confinement coefficient relatively large by the provision of the multi-period super lattice thin film layer. Further, since the number of carriers confined per quantum well layer unit can be increased, it is effective for the low threshold operation of elements.
Further, a multi-period super lattice thin film layer is provided on the upper side of an n-type optical waveguide layer on the lower side of an active layer whereby transportation of electron carriers injected from the n-type optical waveguide layer into the active layer can be adjusted. That is, since an electron carrier is smaller in effective mass and larger in mobility than those of a positive-hole carrier, a transportation speed in the active layer is high. Because of this, since the electron carriers are excessively injected into the active layer, a recouping light emitting progress on which induction release light is based is measured by the injection of positive-hole carriers. In the multi-period super lattice thin film layer provided on the n-type waveguide layer side, the electron carrier assumes a fallen trap state in the supper lattice well layer whose forbidden band width is small. Therefore, the quantity of transportation of electron carriers is relatively small, and it takes relatively long time for transportation. Within the active layer, the recoupling light emitting progress having been measured by the injection of positive-hole carriers is relieved, and the production amount of the optical gain with respect to the injection amount of electron and positive-hole carriers increases. Since this leads to a relative increase in differential gain, operation of elements with low threshold and high efficiency becomes enabled.
Thirdly, when a multi-period super lattice thin film layer is used, when an impurity diffusion area is formed, a diffusion front of the impurity is suppressed and stopped to thereby definitely provide a boundary of the impurity diffusion area.
In a semiconductor laser element, when an optical density is high, a facet of a resonator is broken to limit the maximum optical output. To avoid this, a window structure in which a laser beam is transparent in an area in the vicinity of the end of the resonator is formed. As a method for forming the window structure, there is a process for diffusing impurities to thereby form a multi-quantum well structure active layer in the vicinity of the end of the resonator into a mixed crystal. In the case where the process for diffusing impurities is used, it is important to control the quantity of diffused impurities in the active layer and a position of the diffused front. The multi-period super lattice thin film layer is introduced, and a thin film super lattice layer having an As system is used particularly for a V-group element whereby it can act as a diffusion stop layer of impurities.
Needless to say, it is necessary to enable making small the diffusion coefficient of impurities in which the multi-period super lattice thin film layer is introduced in order to fully function as the diffusion stop layer of impurities. The multi-period super lattice thin film layer has a smaller impurity diffusion coefficient than that of a semiconductor layer above the multi-period super lattice thin film layer to thereby fully function as the diffusion stop layer of impurities. Further, it is effective to introduce a compressed strain. To this end, a compound semiconductor containing arsenic (As) as a constituent element is particularly effective.
For the semiconductor light emitting device, a compound semiconductor material, particularly, a III-V group V group compound semiconductor material is often used. In this case, in a constituent element in which an optical waveguide layer or an active layer is constituted mainly by phosphorous (P), the effect of the multi-period super lattice thin film layer can be exhibited very effectively by making use of a difference in diffusion coefficient. The thin layer multiple super lattice layer is provided on the lower side of the active layer, that is, on the side of a crystal growth substrate whereby the diffusion front at the semiconductor multi-layer can be controlled. For example, in the semiconductor laser device, a mixed crystal of an active layer sometimes occurs by the diffusion of impurities, but such a diffusion has been diffused only in an area as desired, after which the diffusion can be stopped substantially. For example, the impurities are diffused from the top of a multi-layer surface of the semiconductor multi-layer, but the impurities are diffused also in a face direction of the laminated surface of the multi-layer. In this case, at the time when the impurities reach the multi-period super lattice thin film layer, the aforementioned diffusion processing is completed whereby the excessive quantity of impurities diffused on the active layer and an increase of internal optical loss of elements caused thereby can be avoided.
Further, in the case where p-type impurities are diffused, in an impurity diffusion area, a pn homo-junction is formed within an n-type optical waveguide layer in thee past. Therefore, a leak current through the pn-junction occurs. However, since in the present procedure, the diode characteristic of a pin in which the active layer internally comprises an undoped layer can be maintained, it is possible to suppress a turning leak current. Thereby, it is possible to obtain an element of a low threshold current or a low working current which suppresses a leak current as compared with a conventional element formed with a window structure. That is, it is possible to coexist the high output characteristic with the low threshold low current operation by way of the window structure.
By the devise and the design for fully exhibiting the role of the multi-period super lattice thin film layer, it is possible to obtain a light emitting device including semiconductor laser elements provided with the high output characteristic in combination of the low threshold high efficiency operation, which is suitable for a light source for an optical information processing apparatus and an optical communication system device.
It is noted that also in the following examples, as optical feedback means, an example of a so-called Fabry-Perot resonator using a cleavage plane is illustrated. However, the present invention can be carried out for a distributed feedback (DFB) type laser using a diffraction lattice, and a distributed bragg reflector (DBR) type laser.
The semiconductor light emitting device according to the present invention has been produced in consideration of various aspects as mentioned above. This light source is very useful as a light source for an optical information processing apparatus having at least a light source for irradiating light on a recording medium and a detector for detecting a reflecting light from the recording medium and having a function for reading a state change of a part of the recording medium, and a trans- and receive system apparatus for optical communication having at least a light source for transmitting light through an optical fiber as a signal and a detector for receiving a signal from the optical fiber.
Next, various matters as described above are put in order, and various typical forms of the semiconductor light emitting device according to the present invention will be listed (mentioned).
The semiconductor light emitting device according to the present invention includes a semiconductor substrate (a misoriented substrate) having a surface inclined from a fundamental crystalline surface and a periodical semiconductor super lattice thin film layer thereon, wherein at the upper part of the semiconductor super lattice thin film layer, a semiconductor crystalline film is formed with a light emitting layer area and a waveguide path structure.
The fundamental structure of the semiconductor light emitting device will be described in more detail hereinafter. This fundamental structure includes a semiconductor substrate (a misoriented substrate) having a step of an atomic layer order on the surface thereof and a periodic semiconductor super lattice thin film layer having a thickness of an atomic layer order formed thereon, wherein a stepwise non-periodic difference in level (called a step bunching appearing as a value of the atomic layer order) occurring in a growth surface of a semiconductor crystalline film (which is a connection interface when a light source is completed) formed on the semiconductor super lattice thin film layer is suppressed and relieved as compared with that of a semiconductor crystalline film formed on the lower side of the semiconductor super lattice thin film layer, and at the upper part of the semiconductor super lattice thin film layer, the semiconductor crystalline film is formed with a light emitting layer area and a waveguide path structure. That is, the thickness of the semiconductor crystalline layer provided on the semiconductor misoriented substrate is thicker in the vicinity of the step on the surface of the semiconductor misoriented substrate, the step bunching being suppressed or relieved.
The atomic layer order termed herein indicates, concretely, a thickness thinner than a so-called quantum well layer of not more than 20 xc3x85 or a quantum barrier layer, and a dimension equivalent thereto, the desirable range in the present invention being not more than 10 xc3x85 but less than 10 atomic layer.
The semiconductor super lattice thin film layer is preferably of the multi-period super lattice structure. Selection is made such that the period is from 5 to 30, and the whole thickness is from 5 nm to 50 nm. Particularly, in practice, the period is from 8 to 12, and the whole thickness is from 20 nm to 25 nm, which ranges are often used.
In the above-described semiconductor light emitting device, as one example in which the effect of the present invention is exhibited, there is a semiconductor misoriented substrate comprising a substrate material which has a main face in which appears a periodic step of an atomic layer order comprising terraces of (100) surface arranged stepwise and periodically, and whose inclination (an off angle) from the macroscopic (100) surface of the main face is in the range of 0xc2x0 to 54.7xc2x0.
The surface state of the substrate will be described in more detail. The semiconductor misoriented substrate has terraces of (100) surface stepwise, an average angle inclined with the step of the atomic layer order which is off in the range of from larger than 0xc2x0 to smaller than 54.7xc2x0, and a face orientation whose surface is off in the range of 0xc2x0 to 54.7xc2x0 Preferably, the range of the off angle is in the range of 5xc2x0 to 25xc2x0.
As the semiconductor substrate, a substrate of GaAs or InP is practical. These substrate material sometimes contain some impurities, but these can be fully applied to the present invention as a substantial GaAs substrate or InP substrate.
In a semiconductor light emitting device of a GaInP/AlGaInP system in which a substrate material comprises GaAs or InP, the range of the off angle is set to the range of 5xc2x0 to 25xc2x0 whereby the effect of the present invention can be very effectively used in emitting light in a desired red area (600 to 680 nm).
Another example in which the effect of the present invention is particularly effective comprises the constitution of employment of a so-called quantum well type active layer in which a light emitting active layer having a small forbidden band width and an optical waveguide layer having a large forbidden band width sandwiching both upper and lower sides thereof are provided on the semiconductor super lattice thin film layer, and a waveguide path structure according to a use of the semiconductor light emitting device is provided on the optical waveguide layer.
Another form of the present invention provides the semiconductor light emitting device characterized in that a light emitting active layer having a small forbidden band width and an optical waveguide layer having a large forbidden band width sandwiching both upper and lower sides thereof are provided on the semiconductor misoriented substrate, and the semiconductor super lattice periodic thin film layer is provided on the upper side of the optical waveguide layer formed on the lower side of the light emitting active layer and is provided so as to be positioned on the lower side of the light emitting active layer to thereby provide a waveguide path structure of a semiconductor laser light source.
Still another form of the present invention provides the semiconductor light emitting device characterized in that the light emitting active layer is formed with a multi-quantum well structure, and a waveguide path structure in which the semiconductor super lattice periodic thin film layer is provided on the side of the semiconductor misoriented substrate is. constituted in proximity to the quantum well layer first provided on the multi-quantum well structure active layer.
Another form of the present invention provides the semiconductor light emitting device characterized in that in the waveguide path structure in which the semiconductor super lattice periodic thin film layer is introduced by the number of periods corresponding to the optical confinement coefficient for one layer portion of the quantum well layer of the multi-quantum well structure active layer to provide the semiconductor super lattice periodic thin film layer, the waveguide path structure equal in the optical confinement coefficient to that of the case where the quantum well layer of the multi-quantum well structure active layer is reduced by one layer as compared with the case where the semiconductor super lattice periodic thin film layer is not provided.
Still another form of the present invention provides the semiconductor light emitting device characterized in that the multi-quantum well structure constitutes the waveguide path structure in the form of a strained multi-quantum well structure in which at least a lattice strain is introduced into the quantum well layer.
Another form of the present invention provides the semiconductor light emitting device characterized in that the strain multi-quantum well structure is in the form of a strain multi-quantum well structure in which at least a lattice strain is introduced into the quantum well layer, and the quantum barrier layer is in the form of a strain compensation multi-quantum well structure in which a lattice strain having a symbol opposite to that of the quantum well layer is introduced.
In this case, preferably, a quantum energy formed in the semiconductor super lattice periodic thin film layer is set to be larger than a quantum energy in the quantum well structure active layer (including the optical waveguide layer).
It is practically recommended that a combination of the super lattice well layer constituting the semiconductor super lattice periodic thin film layer and a super lattice barrier layer having a larger forbidden band width than that of the former be selected from a group consisting of GaAs/AlGaAs, GaAsP/AlGaInAs, GaInAs/AlGaInAs, GaInAs/AlInAs, GaInAs/AlGaAsP, GaInP/AlGaInP, AlGaInP/AlGaInP, GaInAsP/GaInP, GaInAsP/GaInAsP, GaInAsP/InP, and GaInAs/InP.
The combination of the super lattice well layer constituting the semiconductor super lattice periodic thin film layer and a super lattice barrier layer having a larger forbidden band width than that of the former has been mentioned. In this case, however, preferably, an element As element is contained in either one of the super lattice well layer or the super lattice barrier layer of the semiconductor super lattice periodic thin film layer to thereby constitute a waveguide path structure.
Further, preferably, with respect to a so-called stripe-like waveguide path structure (hereinafter abbreviated as a stripe area) formed in the light emitting active layer or the optical waveguide layer of the semiconductor light emitting device, impurities are diffused and introduced into the light emitting active layer area at the lower part of the stripe area in the vicinity of the end of the resonator to make the forbidden band width of the impurity diffusion area larger than that of the region in which the impurity is not diffused and to make larger than energy of a laser beam generated in the resonator in which the impurity is not diffused so as to form a window structure for releasing the laser beam. Preferably, a difference between the forbidden band width of the area in which the impurity is introduced and the energy of the laser beam generated in the resonator is set to at least not less than 50 meV. Preferably, with respect to the window structure area at the lower part of the stripe, a current non-injection area is provided, at the upper part or lower part thereof, while extending longer into the resonator than the window structure area.